Coalescence Of Helium Bubbles Research Articles

The growth and coalescence of helium bubbles in bicrystal copper under tension

Molecular dynamics (MD) simulations are used to study the growth and coalescence of helium (He) bubbles in bicrystal copper (Cu) on an atomistic scale. The influences of He bubble positions, GB's characteristics, and initial temperatures on the coalescence mechanism are clarified. Our results reveal that the He bubbles distributed perpendicular to the loading direction are easier to coalesce than the parallel ones, and GB will accelerate this process due to the high diffusivity of the GB atoms. A spindle-shaped structure is observed during coalescence for the He bubbles in GB, due to the faster atomic diffusion along the GB plane. Three stages of He bubbles evolution are interpreted during tension loading. For He bubbles distributed in the GB plane, the GBs composed of fewer structural units show a reduced ability to promote the growth and coalescence of He bubbles. What's more, the temperature is found to accelerate the coalescence process due to the faster diffusion of surface atoms at higher temperatures.

Evolution of helium bubbles in SLM 316L stainless steel irradiated with helium ions at different temperatures

The temperature dependence of helium bubble evolution in 316L stainless steel (SS) fabricated by selective laser melting (SLM) has been studied by performing 500 keV helium ion irradiation at temperatures between 350 and 900 °C. The number density of helium bubbles decreased steadily whereas the bubble diameter gradually increased with increasing temperature. The material swelling caused by helium bubbles had no obvious change at the temperature below 800 °C. However, at the maximum temperature of 900 °C, the swelling sharply increased to 0.17% mainly due to the rapidly growing bubble diameter. In particular, heterogeneous nucleation of bubbles occurred in the vicinity of dislocation structures at high temperatures of 700–900 °C, and the formed helium bubbles rapidly grew at the oxide-matrix interface at 900 °C. It was confirmed various intrinsic structures (dislocation structures or oxide particles) differently affected the bubble evolution process. The apparent activation energy (Eact) of helium bubbles determined by the Arrhenius model were used to elucidate the most probable helium bubble evolution mechanisms at various temperatures, as it is an experimentally observed activation energy, which contained information about the activity of helium atoms. It was found that the low-temperature regime with low apparent activation energies (ECbact=0.15eV,ERbact=0.04eV) and high-temperature regime with high apparent activation energies (ECbact=1.50eV,ERbact=0.58eV) favored different helium bubble evolution mechanisms. In the low-temperature regime, helium bubble evolution was controlled by the diffusion of helium atoms via a vacancy mechanism in the SLM 316L SS. On the other hand, the migration and coalescence of helium bubbles also occurred in the high-temperature regime which caused the apparent activation energy of density was higher than the theoretical value of helium atoms diffuse via a replacement mechanism.

In-situ transmission electron microscopy observation of the helium bubble evolution in pre-irradiated fluorapatite during annealing

The polycrystalline fluorapatite Ca10(PO4)6F2 ceramic synthesized by a standard solid-state sintering method was pre-irradiated with 80 keV He+ ions to a fluence of 5 × 1016 ions/cm2 at room temperature. After that, an in-situ annealing experiment was performed inside a transmission electron microscope to monitor the evolution of helium bubbles during heating to 723 and 823 K. Initially, no helium bubble formation was observed in the damage layers of the pre-irradiated samples. However, as the temperature increased, helium bubbles first became visible and then began coarsening, ultimately reaching an asymptotic radius during annealing. The migration and coalescence of helium bubbles in the fluorapatite matrix was complete at a temperature of 823 K, and its likely mechanism involved the existence of two different types of coalescing bubbles.

Segregation and coalescence behavior of helium bubbles in tungsten

We used molecular dynamics to study the segregation and coalescence of helium bubbles in tungsten. We found that, similar to the case of tungsten surface, small mobile helium clusters can segregate and be trapped by helium bubbles. However, the trapping behavior is different for different temperatures. At 700 K, the movement of mobile helium clusters placed near a bubble has a high probability to be confined in the periphery of the bubble, without merging with the bubble. At 1500 K, the mobile He clusters have a high probability to merge with the bubble, forming a single larger bubble. For two immobile helium clusters, it is usually difficult for them to coalesce. In some cases, a single layer of tungsten atoms, or even a single tungsten atom, can prevent two clusters from merging.

Experimental comparison of the (ODS)EUROFER steels implanted by helium ions and irradiated in spallation neutron target

The microstructure of EUROFER and ODS-EUROFER has been characterized by means of positron annihilation spectroscopy (PAS) with a particular focus on the effect of irradiation conditions. Two different irradiation experiments have been used to introduced helium-filled cavities in the studied materials. Materials irradiated in spallation neutron target SINQ have been investigated by conventional positron annihilation spectroscopy, while helium-implanted samples of the same materials have been investigated by slow positron beam technique. With this approach, we were able to obtain a significant amount of positron lifetime and Doppler broadening data and evaluate them with respect to the dpa, irradiation temperature and helium concentration effects. Both types of irradiation experiments confirm a more pronounced creation of growing open-volume defects for the Eurofer steel, comparing to its ODS variant. The bubble-to-void transformation was indirectly but clearly confirmed by the Eurofer specimens. In the case of the ODS Eurofer, the coalescence of helium bubbles, accompanied by saturation of positron mean-lifetime and/or S parameter, was not so evident. This present study provides the first experimental positron annihilation data, showing the onset of void formation in ODS steels.

Molecular dynamics simulations of helium clustering and bubble growth under tungsten surfaces

We study the surface response of W{001} to helium bombardment using molecular dynamics simulations. Simulations have been performed for incident helium of energy 80 eV and surface temperature 2100 K. The saturation of He retention has been observed to be high, a result of the bubbles trapping helium atoms and preventing them from diffusing to the surface and further back into the plasma. On the other hand, we have observe near-surface “cluster rupture” leading to the expulsion of helium atoms towards the vacuum. We have found that bubbles typically grow in a relatively narrow band of He/V ratios (1–3). Besides, it was observed that tungsten atoms migrated from the top surface into the bulk. The coalescence of helium bubbles has also been observed.

Simulation of migration and coalescence of helium bubbles in nickel

The mechanism of migration and coalescence of nanoscale helium bubbles was studied using molecular dynamics. Small helium bubbles in nickel were observed to migrate in a random-walk style at temperatures higher than 0.7 Tm (melting point). Coalescence occurred when two helium bubbles approached each other, leading to the formation of a large bubble. The motion of the helium bubbles in nickel proceeds by displacement of nickel atoms on the surface, which causes the rearrangement of other nickel atoms, resulting surface diffusion. These findings indicate that migration and coalescence may be the possible growth mechanism for small helium bubbles in nickel at high temperature.

Trapping of helium in nano-bubbles in euxenite: Positive identification and implications

The (Y,REE,U,Th)–(Nb,Ta,Ti) oxides, like euxenite, fergusonite, pyrochlore, zirconolite, are known to contain nanometric spherical cavities or bubbles, interpreted to contain radiogenic helium. In-situ analyses by Scanning Transmission Electron Microscopy (STEM) coupled with Electron Energy Loss Spectroscopy (EELS) inside nano-bubbles from an euxenite crystal, sampled in its host c. 920 Ma old pegmatite in Norway, deliver, for the first time, a positive identification of helium and an estimation of helium pressure in such bubbles. The chemically unaltered euxenite crystal proves amorphous and homogeneously speckled with bubbles ranging from 5 to 68 nm in diameter, around a log-normal distribution centered at 19 nm. The euxenite contains 9.87 wt% UO2 and 3.15 wt% ThO2. It accumulated a theoretical alpha-decay dose of 3.46×1020 α/g (i.e. 170 He/nm3), at a dose rate of 11926 α/g/s. This corresponds to production of 0.23 wt% He. The density of helium inside the bubbles, estimated from EELS data, ranges from 2 to 45 He/nm3, leading to a pressure of 8 to 500 MPa. The proportion of produced helium trapped in bubbles is about 10%. Helium bubbles clearly influence helium diffusion. They may contribute to the swelling of euxenite during amorphization and to the fracturing of the host rock. Our results suggest that dose, dose rate and structural state seem to be important parameters for the nucleation, growth and coalescence of helium bubbles but also demonstrate the crucial need of experimental studies to be able to develop a predictive model of the long term behavior of materials in response to helium irradiation. Furthermore, chemical alteration of euxenite, here materialized by fluid driven dissolution–precipitation towards silica bearing euxenite, removes the bubbles and mobilizes helium into the rock via cracks and grain boundaries. It is then suggested that helium-rich fluid released from such U–Th rich sources may percolate into surrounding rock units, inducing perturbation of the (U–Th)/He systematics of apatite and zircon in these units.

Heat and mass transfer of submerged helium injection in liquid oxygen vessel

The submerged helium injection process results in the heat and mass transfer between the helium bubble and the cryogenic liquid. The objective of this paper is to analyze the dynamics of the heat and mass transfer process. It is observed that during the helium injection process the dynamics of mass transfer is dominant and the transient heat transfer is negligible.The helium bubble shape and rising patterns are observed with a visualization device that helps to discern the dominant process between heat transfer and mass transfer. The clustering patterns such as coalescence of helium bubbles are observed with the visualization device. The visualization results indicate that, it is very difficult to determine the representative size of bubbles due to the irregular shape of the helium bubbles. The shape and size of the helium bubbles are important parameters for evaluating the overall mass transfer coefficient (kGA) which is the essential parameter for calculating the evaporation rate of the bulk liquid into the helium bubbles. In this paper, the simplified lumped model is considered to fairly approximate the evaporation rate of the cryogenic liquid into the bubbles and the cooling rate of helium injection. The empirical correlation for the average concentration (C‾A) of evaporated cryogenic liquid into the helium bubbles is presented and the overall mass transfer coefficients (kGA) are calculated as the result of the lumped model. The proposed model and empirical correlations are compared with the experimental results, and the comparison result shows good agreement with differences that are less than±0.4K.

Modeling Space-Time Dependent Helium Bubble Evolution in Tungsten Armor Under IFE Conditions

During Helium implantation or generation in finite geometries, space dependent parameters and features affect Helium transport through the material. Conventional kinetic rate-theory models assume strictly homogeneous field parameters and as such can not directly resolve space dependent phenomena of helium transport. The current work outlines a new approach to simulate space-dependent helium transport during irradiation in finite geometries. The model and the numerical code, called HEROS, are described and applied to simulate typical IFE relevant helium implantation conditions. A case study using the HAPL IFE reactor design is used to demonstrate the capabilities of the HEROS code. It is shown that the HEROS code is capable of simulating very complex transient and space dependent Helium transport in finite geometries, including the simultaneous transient production of defects and space- and time-dependent temperature and temperature gradients. Space dependent nucleation and growth of helium bubbles during implantation are modeled along with the impact of biased migration and coalescence of Helium bubbles.

Model of the Formation and Growth of a Gas-Filled Cavity in Boron Carbide under Irradiation

A model of the formation and growth of gas-filled pores in the cores of absorbing elements under neutron irradiation is described. The model is based on a definite similarity between the mechanisms of radiation damage in boron carbide and oxide fuel. For boron carbide, the key mechanism is radiation-stimulated coalescence of helium bubbles, which gives rise to the much larger swelling of boron carbide as compared with nuclear fuel. Regimes of the coalescence of helium bubbles as a function of the irradiation intensity, neutron spectrum, and radial position are investigated. It is shown that the model describes qualitatively well the basic development of gas-filled porosity under irradiation in fast and thermal reactors.

Coalescence of helium bubbles in aluminum

The formation and coalescence of helium bubbles in aluminum with annealing at 550°C is examined experimentally using electrical resistivity and electron microscopy. Results for the kinetics of coalescence are compared with those obtained by computer simulation in order to test models based upon various mechanisms for bubble motion. The computer method previously developed is extended from an ideal to a Van der Waals gas and effects due to the surface energy of aluminum are considered. Comparison of experimental and computer results indicates that the motion of helium bubbles in aluminum is not surface-diffusion limited and that it is consistent with the facet nucleation-limited mechanism originally proposed by Willertz and Shewmon.